1. Field of the Invention
This invention relates to formulations for oiling leathers and skins containing selected cationic surfactants and to their use for the production of the formulations.
2. Discussion of Related Art
Apart from tanning agents, oiling formulations are the most important auxiliaries for developing the character of leather. Oiling formulations develop their effect by lubricating and thus insulating the leather fibers and by hydrophobicizing the fibers.
Coating the leather fibers with a fatty film reduces mutual friction and, hence, improves the suppleness and elasticity of the tissue. This has positive effects on the tear strength of leather because, in an elastic material, many fibers on exposure to tensile stress align themselves in the direction in which the stress is applied and, in doing so, offer greater resistance to tearing than the same fibers in a brittle material. In addition, tanning effects are obtained through the hydrophobicization because it involves the displacement of water from the skin.
Leather oiling formulations are generally vegetable and animal oils, fats and waxes, the hydrolysis, sulfonation, oxidation and hydrogenation products obtained from these materials by chemical transformation and, finally, mineral oiling formulations; more specifically:
Saponifiable fats and oils and natural waxes and resins belong to the esters. To the leather expert, oils and fats are understood to be esters of glycerol and fatty acids which are solid or liquid at room temperature. From the group of animal fats, train oils, fish oil, beef tallow and neat's foot oil in particular are used for oiling leather; from the group of vegetable fats, castor oil, rapeseed oil and linseed oil in particular are used. In waxes and resins, the fatty acids are esterified with relatively high molecular weight alcohols instead of glycerol. Examples of waxes are beeswax, Chinese wax, carnauba wax, montan wax and wool grease; the most important resins include colophony, birch bark oil and shellac.
The chemical transformation of vegetable and animal fats gives products which are soluble in water and which, in addition, have an emulsifying effect to varying degrees on water-insoluble fats. Known products of this type are, for example, the sulfonated water-soluble oils of various kinds, train oils modified by oxidation (known as Degras or Moellon), the soaps obtained in the hydrolysis of natural fats, hydrogenated fats and, finally, free fatty acids, such as stearic acid, as hot-stuffing fats. Most animal and vegetable fats have a certain affinity for leather which can be considerably increased by the introduction or exposure of hydrophilic groups.
Mineral oiling formulations are also important in the manufacture of leather. These hydrocarbons are similar to natural fats and oils in some properties, but cannot be saponified. They are fractions from the distillation of petroleum which are called mineral oil in liquid form, vaseline in paste-like form and paraffin in solid form.
In many cases, however, unwanted stains are formed with time on the surface of the tanned and oiled leather. This phenomenon is known as fatty spew. Fatty spew is formed mainly on chrome-tanned leathers after relatively short or prolonged storage as a white, often bloom-like coating which covers the surface of the leather either locally or completely. The spew is attributable to the egression of solid fats from the leather. It can be caused by the natural fat basically present in the leather or by fats which have been introduced into the leather during the oiling process.
Fatty mixtures used for oiling leather tend to cause fatty spew in particular when they contain large quantities of free fatty acids. Free fatty acids generally have a higher melting point than their glycerides. The hydrolysis of fats during storage of the leather correspondingly increases the danger of fatty spew.
Soaps and fat liquors are hydrolyzed in chrome leather with release of fatty acids, especially in chrome leather which has not been sufficiently deacidified. Sulfonated oils and fats differ in their tendency to form fatty spew, the tendency to form fatty spew generally decreasing with longer life (cf. Journal of International Society of Leather Trades Chemists 47, 379 (1952).
Fatty spew occurs more easily, the more fats with a tendency towards fatty spew which the leather contains. The quantity, composition and position of the fatty mixture of natural fat and fat liquor present in the leather critically determine the extent and the composition of the spew. Leather with a loose structure is less likely to form fatty spew than leather with a dense fiber structure. Fatty spew is observed more commonly at low temperatures than at relatively warm outside temperatures.
The crystalline fatty spew develops in the hair follicles and glandular channels, small crystals initially being formed low down and gradually filling the entire hair follicle as relatively large fatty crystals, spreading over the surface of the leather and matting together to form a dense crystal film. Any fats containing stearin or palmitin derivatives can cause crystalline fatty spew, the danger of spew formation increasing with increasing concentration [cf. Ledertechn. Rundsch. 1 (1949)].
So-called neutral fats, i.e. substances suitable for oiling leather which do not contain any ionic groups in the molecule, for example fats, waxes and hydrocarbons, have a particular tendency to form fatty spew. Neutral fats in the form of stearin and/or palmitin derivatives, for example corresponding triglycerides or the free fatty acids, are particularly critical in this regard.
Since oiling is in any event an almost essential step after tanning in the processing of leather in order to achieve the required product properties, it has become common practice to use special synthetic oiling formulations with only a minimal tendency to form fatty spew.
A class of oils which have been widely used for this purpose are halogenated compounds, such as chlorinated hydrocarbons. Unfortunately, the increasingly more stringent ecological and toxicological requirements which products entering the environment or coming into contact with the consumer are expected to satisfy make this class of compounds increasingly unattractive.
Besides anionic surfactants, for example sulfonated fats and oils or sulfosuccinates, cationic compounds, for example dimethyldistearyl ammonium chloride, are also important as oiling media. A review of this subject by S. Gupta can be found in J. Am. Leath. Chem. Ass. 83, 239 (1988). However, cationic surfactants are ecologically questionable, in addition to which it is known that, in general, they cannot be used together with anionic surfactants because otherwise salts are precipitated on the surface of the leather. Moreover, their performance properties are not always satisfactory.
Accordingly, the complex problem addressed by the invention was to provide new oiling formulations for leathers and skins based on cationic surfactants which would be distinguished by improved ecological and performance properties including, for example, a more pleasant feel and a lower tendency to form fatty spew.